Sit-up exerciser



FIG. 1 is a front, left side and top perspective view of a sit-up exerciser, showing my new design;

FIG. 2 is a left side view thereof, the opposite side view being a mirror image;

FIG. 3 is a front view thereof;

FIG. 4 is a rear view thereof;

FIG. 5 is a top view thereof; and,

FIG. 6 is a bottom view thereof.

The broken line showing of the handle bar is for illustrative purposes only and forms no part of the claimed design. 

The ornamental design for a sit-up exerciser, as shown and described. 